As you will undoubtedly have noticed, as of the later parts of the anime and especially in Zwei, the relationship between Kazu & Akane becomes the butt of a running gag. However, I'm wondering if there's something else at play here.

Surely, being effectively a walking chastity belt hardly seems to be the best use of one of the Five Columns, so could it be that a sexual union between a Meister Otome and her male Master results in disastrous consequences, which Mahya has been ordered by Natsuki to prevent? Otherwise, there seems to me to be little in the way of a logical reason why Kazu & Akane can't enjoy a sexual relationship and/or get married.

First of all, due to Argos XIV's demise, Kazu is the first ruler of a new dynasty in Chaldea and, as such, it's incumbent on him to provide a successor to secure the future of his kingdom. As it's pretty much impossible to see Kazu with anyone else, Akane seems to be the only logical candidate to become his queen.

Also, from Akane's perspective, though she's only recently joined the ranks of the Meisters, and is stated to have vast potential as an Otome, this problem is hardly insurmountable. Shiho is a native of Chaldea and could easily be reassigned as her current Master King Charles already has Rosalie (who pretty much suggests this scenario in Zwei). Also, being an Otome in the service of Chaldea is pretty much the Mai-Otome equivalent of a Redshirt: we see several turning green & sparkly in the course of the show, with others in the manga (the "superhuman" Layla spectacularly fails to live up to her billing).

Additionally, there are no precedents for romances between an Otome & her Master in the show: the only Otomes who leave the service to have a family are (please add any others if I'm missing any): Fumi (whose Master was a woman); and Lena (who presumably married Shirow).

I'd be really interested to hear any other opinions (or details of the right tree so I can bark up that one) on this.